1. Field of the Invention
The present invention relates to a dielectric filter of a type having first and second internally electroconductive holes defined therein in a juxtaposed fashion by depositing an electroconductive coating.
2. Description of the Prior Art
A prior art dielectric filter is shown in an exploded view in FIG. 6. The prior art dielectric filter shown in FIG. 6 comprises a generally parallelepiped block 61 of dielectric material, first and second coupling pins 62 and a metallic casing 63. The dielectric block 61 has first and second internally conductive holes 61a and 61b defined therein so as to extend parallel to each other and each having an internal surface lined with an electroconductive material, and also has a coupling hole 61c defined therein at a location intermediate between the first and second internally conductive holes 61a and 61b. In this dielectric filter, a coupling capacitance is defined between the coupling pins 62 and the internally conductive holes 61a and 61b.
Also, as shown in FIG. 7 in an exploded representation, in order to render the dielectric filter to have a polarity, internally conductive holes 71a and 71b have to be coupled with each other through a coupling hole 72a; internally conductive holes 71b and 71c have to be coupled with each other through a coupling hole 72b; internally conductive holes 71c and 71d have to be coupled with each other through a coupling hole 72c; and internally conductive holes 71d and 71e have to be coupled with each other through a coupling hole 72d. For the adjustment of each coupling degree, electroconductive patterns 73 are formed on a front surface of the dielectric block where the various holes 71a to 71e and 72a to 72d are opened towards the outside. The frequency of each resonator having the internally conductive holes 71a to 71e is adjusted by applying required electroconductive patterns 74 at the front surface of the dielectric block. The polarity of the dielectric filter can be obtained by forming a coupling capacitance exhibited by connecting the internally conductive holes 71b and 71d with each other through associated coupling pins 75 and an electrode conducting on a printed circuit board (PCB) 76. Alternatively, it is also known that, as is the case with the dielectric filter shown in FIGS. 8(a) and 8(b), internally conductive holes 81a and 81b, internally conductive holes 81b and 81c internally conductive holes 81c and 81d, and internally conductive holes 81d and 81e may be coupled with each other through respective coupling holes 82a, 82b, 82c and 82d, and individual resonators are designed to have a varying length defined by providing steps on a short-circuiting surface 84 so that each of the resonators can have a design resonance frequency.
In the prior art dielectric filters discussed above and shown in FIGS. 6, 7, 8(a), 8(b), since the coupling between each pair of neighboring resonators is determined by the coupling hole, the dielectric filter as a whole has a varying shape for each type, that is, each resonance frequency and/or each bandwidth and, therefore, the use of different molds is required for each type of the dielectric filters. In terms of manufacture, the presence of the coupling holes which are non-plated areas makes it difficult to mass-produce electrodes by the use of an electroless plating technique. Due to the problem associated with the mass-productivity, the prior art dielectric filter are disadvantageous in terms of cost. Also, due to the presence of the coupling holes, the dielectric block used in any of the prior art dielectric filters tends to become bulky and this is in contradiction to the demands for miniaturization of the dielectric filter.